I have seen quite a few posts with questions about how to stop the concrete from going down all the block webs and decided to post this. This is what we use on the job site its very inexpensive (not cheaper than already empty concrete bags or newspaper for that matter) but its an alternative for those who choose to use it.

Here is the product description, most HD or LWs have it or of course your masonry supply.

Grout Stop ™
Designed to prevent grout from falling through block core while maintaining positive bond in mortar joint.

The pic is actually misleading, when this is put in its between 2 courses and not covered with mortar as shown in the pic. The web is put in place then your bed joint goes on it then the block. For the oven construction putting it below the last course would give you (sort of) a bond beam all the way around the top under the hearth and key in the hearth slab to the base. which regardless of where you are building is a good thing...

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